There is a continuing need for a safe and effective surgical snare apparatus for the removal of protruding tissue such as tonsils, warts, polyps and the like from the body. Conventional surgical snares operate by pulling a conductive wire loop, surrounding a tissue protrusion, into a tubular sheath. These surgical snares are typically modified to allow connection of an electrical source to the conductive wire loop, and the tubular outer sheath is typically made of a non-conductive material. When the wire loop is snug around the tissue to be removed, high frequency electrical current is delivered to the tissue in contact with the conductive wire loop from an appropriate electrosurgical generator. The high frequency electrical current is concentrated on the tissue at the point of contact of the wire loop; it travels through the body at lower concentrations, to a large surface area dispersive electrode, and returns to the electrosurgical generator, thus completing the circuit.
For a conventional electrosurgical snare to function properly, the critical requirement is a proper current density at the interface between the wire loop and the body tissue. This current density must be sufficiently high to allow effective heating of the intra cellular fluid, drying the tissue to prevent hemorrhage. As a tissue in contact with the snare wire is coagulated, its electrical resistance increases. The conventional electrosurgery generator, to which the snare is attached, is capable of producing voltages as high as several thousand volts. This allows energy to arc across the high resistance tissue, resulting in electrosurgical cutting through the dry tissue. A user, skilled in the practice of this technique, tries to maintain the critical balance between the rate of tissue drying and the rate of high voltage arcing for electrosurgical cutting. The desired clinical result is the removal of the tissue without hemorrhages at the site of removal.
There are numerous disadvantages to the conventional electrosurgical snare. It is difficult for the user of a conventional snare to maintain the correct level of electrosurgical current density with varying size growths. For instance, if the snare loop is fully extended around a large piece of tissue, initial electrosurgical power levels considerably higher than typically used will be required to obtain the proper current density, along the interface betwen the wire loop and the tissue, to start the procedure. As this procedure proceeds, and the wire loop is pulled back into the tubular sheath, the diameter of the exposed wire loop decreases, resulting in a potentially dangerous increase in current density along the remaining exposed wire to tissue interface. This results in a rapid increase in the rate of electrosurgical cutting of the tissue; and also results in a corresponding decrease in the quality of coagulation along the resected surface. This can result in dangerous hemorrhage of the patient. Additionally, these higher initial power levels cause higher current densities at other points along the electrical pathway between the wire loop and the dispersive electrode in contact with the patient's body at another site. This can result in undesirable damage to other body organs and tissues.
Although this conventional type of electrosurgical snare is in wide use, it is impossible for the user to avoid the increase of the current density along the exposed wire loop as the loop is pulled into its sheath during the course of the removal of tissue. Because of the varying initial size of tissue growths to be removed, and the changing effective length of the electrically conductive wire during a procedure, it is very difficult to select an initial electrosurgical power level that will provide consistently predictable clinical results. Furthermore, since the electrical currents must travel from the contact point of the snare wire to the dispersive electrode for return to the electrosurgical generator, it is not possible to predict the damaging effects along the random tissue pathway involved. Each of the risks associated with the use of a conventional snare increases as the size of growth to be removed increases.
The conventional electrosurgical snare described above is also known as a monopolar electrosurgical device. The monopolar description applies since only one portion of the electrical circuit, namely the snare wire to tissue interface, is controlled by the user. The second portion of the electrical circuit, namely the dispersive electrode, is at a site removed from the desired site of electrosurgical effect.
To reduce the risks associated with the conventional monopolar electrosurgical snare, attempts have been made to design bipolar electrosurgical snares. In a bipolar electrosurgical device, two electrically separated contacts are incorporated in a single device controlled and maneuvered by the user. One contact acts as the active electrode, the second contact acts as the return electrode. This isolates the electrosurgical effect to the tissue between the two contacts, leading to the conclusion that bipolar electrosurgical devices are intrinsically safer than monopolar electrosurgical devices. The possibility of an unintentional electrosurgical effect at a site removed from the point of contact of the bipolar device is eliminated.
Attempts have been made to incorporate this bipolar concept into the design of electrosurgical snares for the removal of tissue from the body. One such device, U.S. Pat. No. 4,311,143, incorporates a small conductive surface on the exterior of the outer tubular sheath of the snare mechanism to act as the pathway for return of the electrosurgical current to the generator. In this device the wire loop forms the other portion of the electrosurgical circuit. A significant disadvantage of this design, as with conventional monopolar snares, is that the current density along the active wire loop remaining in contact with the tissue growth increases as the loop is withdrawn into the sheath and the diameter of the loop remaining in contact with tissue decreases. This can lead to the same problem associated with monopolar snares, namely, an increasing rate of cutting and a decreasing degree of coagulation with the potential of hemorrhage. Another disadvantage of this design is that the conductive surface acting as a return electrode on the outer surface of the sheath can cause an electrosurgical effect to the normal tissue in an area adjacent to the abnormal tissue being removed rather than isolating the effect to the surface from which the abnormal tissue is being removed. Recognizing that electrical currents will always seek the pathway of least resistance, another disadvantage of this design arises. If the active wire loop is surrounding a large piece of tissue the apparent pathway for electrosurgical current will be through the tissue in contact with the portion of the wire loop closest to the tip of the sheath to the electrode surface on the sheath of this snare. This prevents the desired effect from occurring to the tissue in contact with the portion of the wire loop that is furthest from the tip of the snare sheath. Attempts to increase the electrosurgical effect along the entire portion of the exposed loop of this design by increasing the power level of the generator will only further compound the problem of the pathway of least resistance being sought by this energy. As with conventional monopolar snares, the disadvantage of this design is most obvious when attempts are made to remove larger growths.
Another attempt to incorporate the concept of bipolar electrosurgery into a snare is shown in U.S. Pat. No. 4,493,320. In this design, a loop is formed by two separate conductive wires joined together by an electrically non-conductive material to form a loop. These two electrically conductive wires are electrically separated as they enter the end of the sheath since the sheath has a double lumen. In this device, the entire length of the two wires forming the sides of the loop are electrically conductive. A significant disadvantage of this design, as with conventional monopolar snares, is that the current density along the wire remaining in contact with the tissue growth increases as the wires forming the loop are withdrawn into the sheath and the diameter of the loop remaining in contact with tissue decreases. This can lead to the same problem associated with monopolar snares, namely, an increasing rate of cutting and a decreasing degree of coagulation with the potential of hemmorhage. Once again, recognizing that the pathway of least resistance will always be sought by the electrical currents involved, it is easy to recognize that the greatest electrosurgical effect to tissue captured within this type of loop will occur in the area immediately adjacent to the face of the double lumen sheath, and in the area immediately adjacent to the face of the insulating material that joins the distal ends of the wires to form the loop, since the conductive wires are physically closest together at these points. As soon as adequate drying of the tissue in these areas occurs, the high voltages presented by the typical electrosurgery generator will cause arcing in these areas. This arcing can prevent adequate electrosurgical coagulation from occuring to the remaining tissue surrounded by the extended loop.
In summary, conventional electrosurgical snares do not provide for safe, consistent and predictable electrosurgical results when a given nominal level of electrosurgical current, supplied by the complementary electrosurgical generator, is applied to abnormal tissue growths of varying sizes. By the very nature of the construction of conventional snares, the size of the electrically active loop changes by several magnitudes as the procedure of tissue resection proceeds, leading to the possibility of a dangerously high current density along the remaining exposed loop surface.